1. Field of the Invention
The present invention relates generally to the field of magnetic recording disk drives, and more particularly, to a preamplifier and method for synchronizing write pulses with bit islands in discrete bit patterned recording media.
2. Description of the Related Art
Magnetic recording disk drives for data storage conventionally use disks coated with a continuous layer of magnetic alloy materials or media, rotated at high speed using a spindle motor. Data bits are written in radially-separated concentric tracks on the surface of the spinning magnetic medium by applying a magnetic field using an inductive write head that is driven by current pulses from a write driver circuit to magnetize regions in the medium. The write driver and associated write data path circuitry are commonly integrated into a preamplifier chip together with read channel preamplifier circuitry. The areal density of data, corresponding to the size of bits that may be written, is physically limited in continuous media by cooperative effects that are a combination of magnetic domain size and alloy material grain size. Typically several tens of grains are used per bit to obtain noise reduction by averaging over grain positions and orientation, in order to limit so-called transition noise. Reducing the size of a bit by using a smaller number of grains per bit increases transition noise unacceptably. Reducing the size of the grains likewise runs into a limit called the superparamagnetic limit, in which smaller grains are susceptible to thermal instability of their magnetization. Patterning media into small isolated blocks or “bit islands” of magnetic material separated by nonmagnetic areas has been proposed to overcome these limits. Each bit island can consist of a single grain or of a few strongly-coupled grains that switch magnetization as a single domain, thereby avoiding transition noise. These media, in which individual bits are physically isolated and separated along each circumferential track as well as radially, are sometimes referred to as bit patterned media, to distinguish them from earlier patterned media technologies that created separate tracks, but did not separate bits along a track. It is projected that the use of bit patterned media will be needed at data densities of approximately one terabit per square inch, which will be reached in the next few years given current trends in magnetic recording density.
In conventional magnetic recording disks with continuous media, a read clock signal is recovered from the readback data, typically using a phase-locked loop (PLL) clock recovery circuit within the digital read channel chip in the drive controller, to be used in sampling the read data. There is no need to synchronize the writing of data with the position of continuous media since all of the media contains magnetic material. However, for bit patterned media, the locations of the bits to be written are predefined as the positions of the bit islands, and the write head needs to be pulsed to precisely switch the write fields as the head passes over each bit island. Using a conventional unsynchronized write clock is not effective because imperfections in media patterning, spindle speed stability, disk eccentricity, and temperature gradients lead to varying intervals between data islands. A slip in the phase of write pulses can cause the attempted writing of data to nonmagnetic areas between the bit islands, resulting in a high bit error rate for the write/read cycle. Thus closed-loop control of the frequency and phase of the write clock is required to correct the timing of write pulses.
These issues have been recognized, and various approaches and circuits for sensing the position of bit islands, and for generating and adjusting timing signals and programmable delays to be used in write synchronization have been proposed. In most cases, these approaches attempt to align the write pulses by applying a delay to the write data presented to the write driver, but fail to fully account for variability in electronic and physical propagation delays in the clock recovery function and in the write data path between the delay generator and the write head. Thus there remains a need for improved methods of synchronization of writing to bit patterned media, as well as preamplifier architectures and designs optimized so as to minimize sources of timing inaccuracies.